The basic structure and operation of a cellular radiotelephone system has been disclosed in a variety of publications. See, for example the January 1979 issue of The Bell System Technical Journal; and Specification EIA IS-3-B entitled "Cellular System Mobile Station-Land Station Compatibility Specification" (July, 1984, Electronic Industries Association).
As is well known, the process called "hand off" is a fundamental part of the cellular radiotelephone scheme. A simplified cellular radiotelephone system 10 is shown in FIG. 1. Cellular system 10 includes several fixed RF transceiving stations 12 each serving an associated discrete geographical area ("cell") 14. A central controller 16 supervises and controls the operation of the fixed stations 12. As a mobile station 18 moves from a first "cell" (e.g., cell 14B) to a second "cell" (e.g., 14C), the central controller 16 controls the fixed station 12B serving the first cell 14B to discontinue handling the mobile station's call and controls the fixed station 12C serving the second cell 14C to begin handling the call (and also controls the mobile station to retune to a frequency fixed station 12C operates on). In this way, the mobile station 18 (and its call) is "handed off" to the cell receiving the strongest signal from the mobile station. High quality communications is thus maintained even while mobile station 18 is moving from one cell to another.
System 10 measures the RF signal strength of transmissions of mobile station 18 at the locations of fixed stations 12 in order to decide when a hand off should occur. Decreased received signal strength at a fixed station 12 indicates that the mobile station 18 transmitting the signal is nearing the edge of the cell 14 served by the fixed station and is likely to need handing off to a different cell. Signal strength measurements performed by fixed stations 12 serving adjacent cells are used to determine which cell the call should be handed off to (the call is generally handed off to the cell receiving the mobile station transmission at the highest received signal strength), thus maximizing communications quality and reliability and minimizing the number of hand-offs necessary.
When system design includes partitioned cells (pie-shaped sectors, overlayed cells, etc.), signal strength measurements at fixed stations may also be used to determine which cell partition may best serve particular mobile stations. Signal strength measurements using mobile equipment may be used to verify the RF field strength pattern of fixed station transmissions for purposes of propagation analyses.
As will be appreciated, signal strength measurements are very important in the design and operation of cellular radiotelephone communications systems, and are indeed an essential requirement of cellular equipment design.
Every hand off in a cellular radiotelephone system requires a number of signal strength measurements. Since cellular systems typically serve large numbers of mobile stations, many signal strength measurements are required. Moreover, because mobile stations are usually in motion, the cellular system must respond very rapidly to changes in received signal strength (e.g., by handing off calls) to maintain acceptable signal levels as mobile stations move from cell to cell. There is therefore a great need for fast and accurate received signal strength measuring techniques.
RF signals transmitted by mobile radio stations are subject to Rayleigh fading, as is well known. Fades are of short duration and may be twenty dB or more below the average received signal strength level, making accurate and rapid signal strength measurements difficult to obtain (a measurement made during a deep fade is not representative of the true average received signal strength).
Prior art methods of overcoming this difficulty include analog filtering (equivalent to damping a meter movement so that it does not respond to fast transients) and mathematical averaging of a number of measured samples of received signal strength. Such prior art techniques require several measurements to be taken over a period of time large enough to mask the effects of fading. The number of samples averaged using the averaging technique must be great enough so that measurements made during fades do not unduly influence the resulting average.
Such prior art techniques suffer from at least two disadvantages. First, the extended time period required to obtain accurate measurements using such techniques in is conflict with the requirement that received signal strength measurements must be made as rapidly as possible. Second, it is often desirable to be able to measure sudden changes in the average signal level, such as when the mobile passes behind a large obstacle which "shadows" the antenna. For a rapidly moving vehicle, these changes may occur only a little more slowly than the Rayleigh fading which it is desirable to mask.
Both the averaging and damping techniques of the prior art tend to mask these rapid signal strength changes along with the received signal strength changes attributable to Rayleigh fading phenomenon. As the "damping" (or the number of samples being averaged) is increased to overcome fading effects received signal strength measurement becomes insensitive to other fluctuations in received signal strength which it may be helpful or desirable to measure. As a result the cellular system may respond too slowly to changes in signal strength, allowing the mobile station to receive unacceptable service quality and perhaps even causing the loss of service. Even more important, the excess time required for measurement reduces the number of mobile stations which can be handled, i.e., additional equipment is required to increase system capacity.
The technique disclosed in U.S. Pat. No. 4,549,311 to McLaughlin (Oct. 22, 1985) measures the strength of a RF signal by sampling the signal two or more times during a predetermined time interval and selecting the sampled signal strength having the largest magnitude. The method taught by this McLaughlin patent is essentially a digital implementation of a peak reading meter. The McLaughlin technique always chooses the largest of a plurality of samples (i.e., the peak received signal strength), and therefore is insensitive to received signal strength fluctuation attributable to the values of other measurements in the sampling interval. The technique will begin to detect rapid changes in the average only when a new sampling interval is obtained.
There is great need for an accurate high-speed received signal strength measuring technique which masks the effect of Rayleigh fading, but which is sensitive to received signal strength changes caused-by effects other than Rayleigh fading (e.g., obstacles in the signal transmission path of a moving mobile station).